Electron beam devices, in particular a scanning electron microscope (also referred to in the following text as SEM) and/or a transmission electron microscope (also referred to in the following text as TEM), are used to examine objects (samples) in order to obtain knowledge about the characteristics and behavior of objects in specific conditions.
In the case of an SEM, an electron beam (also referred to in the following text as a primary electron beam) is produced using a beam generator and is focused by a beam guidance system onto an object to be examined. The primary electron beam is guided in a raster shape using a deflection device over a surface of the object to be examined. The electrons in the primary electron beam in this case interact with the object to be examined. As a consequence of the interaction, in particular electrons are emitted from the surface of the object to be examined (so-called secondary electrons) and electrons in the primary electron beam are scattered back (so-called back-scattered electrons). The secondary electrons and back-scattered electrons are detected and are used for image production. An image of the surface of the object to be examined is thus obtained.
Furthermore, it is known from the prior art for combination devices to be used to examine objects, in which both electrons and ions can be passed to an object to be examined. By way of example, it is known for an SEM to additionally be equipped with an ion beam column. An ion beam generator which is arranged in the ion beam column is used to produce ions which are used for preparation of an object (for example etching of the object or application of material to the object), or else for imaging. In this case, the SEM is used in particular to observe the preparation, or else for further examination of the prepared or unprepared object.
In the case of a TEM, a primary electron beam is likewise produced using a beam generator and is passed to an object to be examined using a beam guidance system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons in the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a phosphor screen using a system comprising an objective and a projection lens, or are detected by a position-resolving detector (for example a camera). In addition, for this purpose, it is possible to provide for back-scattered electrons on the object to be examined and/or secondary electrons emitted from the object to be examined to be detected using a further detector, in order to image an object to be examined. The abovementioned imaging is in this case carried out in the scanning mode of a TEM. In this case, the primary electron beam of the TEM is focused on the object to be examined, in a similar manner to that in the case of an SEM, and is guided in a raster shape over the object to be examined, using a deflection device. A TEM such as this is generally referred to as an STEM. Analogously, in the case of an object to be examined through which radiation can be passed, an SEM can also be operated as an STEM.
By way of example, it is possible to use the TEM to determine the structure of a crystal in more detail. For this purpose, in particular, the images or diffraction patterns produced using the electrons passing through the object are evaluated. In principle, a diffraction pattern is a Fourier transform of the object to be examined and has structures whose position in the diffraction pattern is governed by the distances and spatial frequencies of the lattice structure of the crystal. It is also possible to use the intensity of the structures to make deductions about the content of an elementary cell of a crystal. The abovementioned diffraction pattern can be obtained without a scanning mode, or in the scanning mode of the TEM.
In the prior art, a standard raster process is carried out for the scanning mode of a particle beam device. In this standard raster process, a raster pattern is predetermined having a multiplicity of raster lines which are arranged parallel to one another, with each of the multiplicity of raster lines comprising an identical number of raster points (pixels). The primary electron beam in the standard raster process is guided to a first end of a first raster line. The primary electron beam is then guided, starting from the first end of the first raster line, in the direction of a second end of the first raster line from one raster point to another, until the second end of the first raster line is reached. The primary electron beam is then guided to the first end of a second raster line. An identical process is carried out with the second raster line to that with the first raster line. During the process, the time for which the primary electron beam remains at each raster point is identical, and can be predetermined in a fixed form by a control system for a specific imaging mode.
By way of example, the imaging mode defines a chosen magnification and the speed at which the primary electron beam is scanned over the object. During the abovementioned guidance of the primary electron beam from the second end of the first raster line to the first end of the second raster line, it is known from the prior art for a further dwell duration to be provided. This further dwell duration is predetermined in a fixed form, for example by the control system, as a function of a chosen imaging mode. A fixed flyback time is predetermined as a function of the chosen imaging mode, with this being the time which is intended to exist between reaching and dwelling at the second end of the first raster line on the one hand and the guidance of the primary electron beam to the first end of the second raster line, on the other hand. A further known embodiment provides for the primary electron beam to be guided from the second end of the first raster line to the first end of the second raster line, where the primary electron beam remains during the further dwell duration, until the primary electron beam passes over the second raster line, following a trigger signal. By way of example, this trigger signal is coupled to the power supply system frequency.
The guidance of the primary electron beam from one raster point to a next raster point is also referred to as raster guidance or else rastering.
In a high-resolution mode, an STEM can be used to make atomic structures visible, and to image them. Furthermore, it is also possible to image a spatially periodic crystal structure of crystalline samples. During the evaluation of images such as these, the Fourier transform is used to represent the spatial frequencies present in a respective image. The intensity of a point in a Fourier-transformed image (also referred to in the following text as an FFT image) of the respective image indicates the amplitude with which a specific spatial frequency (periodicity based on frequency and direction) is present in the respective image.
However, it has been found that disturbances which influence the image of very small structures (for example atomic structures of a crystalline object) lead to artifacts in the respective image. Even minor periodic disturbances (for example fluctuations in the position of the primary electron beam, fluctuations in the intensity of the primary electron beam and/or fluctuations in the intensity of a detection signal) can lead to undesirable artifacts being visible both in the respective image and in the corresponding FFT image. The artifacts make it harder to evaluate the FFT image.
Accordingly, it would be desirable to provide a method and a particle beam device for producing a representation of an object, in which images which are produced, in particular also FFT images, are as free as possible of artifacts which are not caused by the object to be examined.